Underwater pipe-laying apparatus and method

ABSTRACT

A pipe-laying apparatus and method for laying or raising a pipeline from a vessel. The apparatus includes a first pipe gripping assembly disposed on a pipe laying path along which pipeline passes during laying, a second pipe gripping assembly disposed on the 10 pipe-laying path, and a drive unit for causing relative movement in a direction along the pipe-laying path between the first pipe gripping assembly and the second pipe gripping assembly. The first pipe gripping assembly includes both a pipe gripping arrangement for gripping a pipe during pipe-laying and a cable gripping arrangement for gripping a cable during an operation when a cable is connected to a length of pipeline.

BACKGROUND

This invention relates to a pipe-laying apparatus for laying a pipelinefrom a vessel, to a vessel including such an apparatus and to a methodof conducting pipe-laying operations employing such an apparatus.

In shallow water “S”-laying is commonly used when laying pipeline,whilst in deeper water “J”-laying is more common. When laying pipelineby J-laying, various methods for holding the heavy weight of the pipe asit is lowered from the vessel along a tower are employed. In one knownapproach a combination of a travelling block and a pedestal is usedwhilst in another approach crawler track tensioners are used.

As a further safety measure, it is also known to provide additionalsafety clamps for gripping the pipe in the event that the usual grippingand lowering arrangement is inadequate or inoperative. Two safety clampsmay be provided, one being movable a short distance along the tower andthe other being at a fixed position so that by a suitable sequence ofopening and closing the clamps and moving the movable clamp the pipelinecan be raised or lowered, although at a much slower rate than duringnormal practice. Furthermore during laying of a pipeline from a vesselit may be necessary to lower an end of the pipeline into the water toabandon the pipeline or to recover the end from below water level. Ineither case a cable is connected to the pipeline by means of a specialhead which is welded to the pipeline, and commonly a winch of some kindis employed to raise or lower the pipeline. Where reference is made to a“cable”, it should be understood that the term “cable” is to beinterpreted broadly and may include any form of flexible tension bearingmember, including a fibre rope or a steel wire rope, for example, “fullsection” rope.

In either case of abandonment or recovery of the pipeline the cable fromthe winch has to accommodate very high tension loads. Often a doublecapstan winch and spooling device is used. Such an arrangement does not,however, avoid the need for a very heavy duty winch and results in longlengths of cable bearing very high tension loads, particularly when thecables run parallel to the deck, which is undesirable from a safetyviewpoint. The problem is especially severe when laying a pipeline indeep water, especially when using a J-laying technique, because thatresults in even higher tension loads. It is then usually advantageous tohave the cables extending along the tower down which pipe is laid.Providing such a winch arrangement in the tower presents significantfeasibility and cost issues, because of the high loads involved, andbecause of the size of the winch and the need to provide adequate spaceand structural support in or near the tower to accommodate the winch. Itis an object of the invention to provide a pipe-laying apparatus and amethod of conducting pipe-laying operations that overcomes or mitigatesthe problems caused by using a winch for abandonment and recoveryoperations.

SUMMARY

According to the invention there is provided a pipe-laying apparatus forlaying a pipeline from a vessel, the apparatus including

a first pipe gripping assembly disposed on a pipe-laying path alongwhich pipeline passes during laying,

a second pipe gripping assembly disposed on the pipe-laying path,

a drive unit for causing relative movement in a direction along thepipe-laying path between the first pipe gripping assembly and the secondpipe gripping assembly,

the first pipe gripping assembly including both a pipe grippingarrangement for gripping a pipe during pipe-laying and a cable grippingarrangement for gripping a cable during an operation when a cable isconnected to a length of pipeline

By providing a movable pipe gripping assembly that is capable ofgripping both a pipe and a cable, it becomes possible to provide asingle piece of equipment which can serve the purpose of both the safetyclamps referred to above and the winch system referred to above. Thatenables an apparatus that is both safer and more economical to beprovided.

Where reference is made to “pipe-laying” it should be understood thatthe term is to be interpreted broadly and may include lowering apipeline, raising a pipeline, abandoning a pipeline and recovering apipeline.

Whilst the invention may be applied to S-laying, it is of particularvalue when J-laying because of the higher loads that are experiencedduring J-laying. Thus the portion of the pipe-laying path along whichthe first and second pipe gripping assemblies are disposed is preferablyupright. It will be understood by those skilled in the art that inJ-laying, the pipeline may be lowered from the vessel in a substantiallyvertical orientation or at a significant inclination to the horizontal.Thus the term “upright” used herein should not be taken to mean verticalor even almost vertical but may include angles inclined by up to 45degrees to the vertical.

The first and second pipe gripping assemblies may each be arranged to bemovable along the pipe laying path. In that case, a second drive unitmay be provided, each drive unit being able in use to cause movement ofa single respective pipe gripping assembly. It is however preferred thatonly one of the first and second pipe gripping assemblies is movable bythe drive unit in a direction along the pipe laying path. In that case,the pipe gripping assembly which is movable, may in principle, be aboveor below the other pipe gripping assembly but preferably it is below theother pipe gripping assembly.

As is clear from above, it is not necessary for both pipe grippingassemblies to be movable along the pipe-laying path and for simplicityit is preferred that one of the pipe gripping assemblies is at a fixedposition along the pipe-laying path.

Where the pipe-laying path is upright, the pipe gripping arrangement ofthe first pipe gripping assembly may be provided below the cablegripping arrangement of the first pipe gripping assembly. It is in anycase preferred that the pipe gripping arrangement of the first pipegripping assembly is displaced along the pipe-laying path from the cablegripping arrangement of the first pipe gripping assembly.

Although the first pipe gripping assembly includes both a pipe grippingarrangement for gripping a pipe during pipe-laying and a cable grippingarrangement for gripping a cable during an operation when a cable isconnected to a length of pipeline (for example, during an abandonment orrecovery operation), the two gripping arrangements are preferablyoperable independently of one another. Thus it is preferred that thepipe gripping arrangement for gripping a pipe is able to be changedbetween a gripping condition for gripping a pipe and a releasingcondition in which the pipe is free to pass through the pipe grippingarrangement, that the cable gripping arrangement for gripping a cable isable to be changed between a gripping condition for gripping a cable anda released condition in which the pipe is free to pass through the cablegripping arrangement, and that the changing of the pipe grippingarrangement between a gripping condition and a released condition isable to be effected independently of the changing of the cable grippingarrangement between a gripping condition and a released condition.

The invention may employ any of a wide variety of pipe grippingarrangements and cable gripping arrangements. For example, if a pipelineis provided with circumferential collars or other formations atsufficiently frequent intervals along the pipeline, the grippingarrangement may “grip” the pipe by engaging the side faces of thoseformations. In that case the pipe gripping arrangement need not pressinwardly on the pipe. Usually, however, it will be preferred that thepipe gripping arrangement includes a plurality of pipe contactingelements movable radially inwardly to grip the pipe. The pipe contactingelements may rely exclusively on friction to grip but it is preferredthat the elements include teeth for engaging the outer surface of thepipe. The outer surface of the pipe may be provided with a pipe coatingto reduce corrosion, for example having a thickness of 2 to 3 mm,depending on the use of the pipe and other factors. The outer surface ofthe pipe may be provided with a further coating layer. The furthercoating layer may, for example, be between 1 and 3 mm thick, andcomprise a plastics material, for example, polyethylene orpolypropylene. The provision of a further coating layer may allow onlythe further coating layer to suffer permanent indentation as a result ofthe contacting elements. This, in turn, allows the pipe coating toreduce corrosion to remain intact, and the gripped pipe may still formpart of the pipeline. It is preferred that the teeth are arranged, whenthey grip the pipe, to make permanent indentations only in the furtherpipe coating if safe and possible during use so that thecorrosion-reducing pipe coating layer remains undamaged. The teeth arepreferably arranged to make permanent indentations in the pipe coating,or the pipe wall, only if necessary during use, for example in a casewhere indenting the further coating provides insufficient grip toprevent slippage of the pipe relative to the teeth. The further pipecoating may be contiguous with the corrosion reducing layer, such anarrangement being equivalent to a pipe with an extra thick pipe coatinglayer. Whilst the making of such permanent indentations may not begenerally desirable during normal pipe-laying operations, it is of muchless significance during operations such as abandonment and recoveryoperations and emergency operations where the extra grip afforded by thepenetration of the contacting elements into the pipe wall or pipecoating is of particular advantage. Also, the extra grip may be used inother circumstances for example to allow high loads to be spread betweenthe pipe gripping assembly and other pipe tensioning means. The lengthof pipe which has been indented by the clamp teeth may be damaged by theoperation of the clamp and may ultimately have to be cut off thepipeline and substituted with a new length of pipe. Alternatively oradditionally a piece of pipe can be joined to the special abandonmentand recovery head mentioned above.

The pipe gripping arrangement further includes an activating elementmovable, in use, between an activated position in which there isfrictional engagement between the activating element and a pipe and adeactivated position in which the activating element does not preventfree passage of pipe through the pipe gripping arrangement.

The pipe gripping arrangement, activating element and pipe contactingelements are arranged such that when, during use the activating elementis in the activated position, the pipe contacting elements are driveninto greater engagement with the pipe by movement of the pipe, relativeto the pipe gripping assembly, along the direction of laying. Such anarrangement allows for progressive gripping of the pipe. When theactivating element is in an activated position, the pipe is at leastpartially supported by the frictional force between the activatingelement and the pipe. The load of the pipeline on the pipe is a downwardload. If the load of the pipeline on the pipe is sufficient to overcomethe frictional force between the activating element and the pipe, thepipe will begin to slip and, as a result of the arrangement of the pipecontacting elements, the activating elements and gripping members areforced towards the pipe and the teeth of the pipe contacting elementsmake indentations in the outer surface or outer coating of the pipe. Ifthe pipe continues to slip, the gripping members continue to be forcedtowards the pipe and the teeth of the pipe contacting elements mayengage with, and make indentations in, the pipe wall. Such aself-tightening arrangement helps to ensure a strong grip on the pipe,when necessary.

The above described pipe-gripping arrangement may have utilityindependently of the cable gripping arrangement.

The invention thus further provides a pipe gripping arrangementcomprising a plurality of pipe contacting elements movable radiallyinwardly to grip a pipe. The pipe gripping arrangement may furtherinclude any of the features described herein. For example, the pipegripping arrangement may be provided with teeth that cause, in use,indentations on the pipe, which may for example include a further pipecoating.

The cable gripping arrangement preferably includes a plurality of cablecontacting elements movable radially inwardly to grip the cable. Thecable contacting elements are preferably mounted in the cable grippingarrangement for limited movement in a direction inclined at an acuteangle to the pipe laying path and directed inwardly along the directionof laying, whereby the cable contacting elements are driven into greaterengagement with the cable by movement of the cable along the directionof laying. Such a self-tightening arrangement helps to ensure a stronggrip on the cable.

Usually, because the apparatus is intended for use in special situationsonly, the stroke of the drive unit, which may for example comprise oneor more hydraulic piston-and-cylinder arrangements, will be much smallerthan the stroke of, for example, a travelling block used during normallaying. Thus the stroke of the drive unit may be less than one length ofpipe. The stroke may be less than 18 m and is preferably less than 12 m.

The second pipe gripping assembly preferably includes both a pipegripping arrangement for gripping a pipe during pipe-laying and a cablegripping arrangement for gripping a cable during an operation when acable is connected to a length of pipeline. The second pipe grippingassembly may have any or all of the features defined above in respect ofthe first pipe gripping assembly. Preferably the second pipe grippingassembly is similar to, and more preferably substantially the same as,the first pipe gripping assembly.

The cable gripping arrangement of one of the first and second pipegripping assemblies may be arranged to fit partially inside the pipegripping arrangement of the other of the first and second pipe grippingassemblies such that the stroke length of the cable gripping arrangementis slightly longer than the stroke length of the pipe grippingarrangement.

The present invention also provides a pipe-laying vessel including apipe-laying apparatus as defined above. The pipe-laying apparatus ispreferably mounted on a tower of the vessel. The tower may be pivotallymounted to adjust the inclination of the tower to the vertical.

The present invention further provides a method of conductingpipe-laying operations employing a pipe-laying apparatus as definedabove.

As will be understood from the description above, the operation mayinvolve feeding of a pipe through the apparatus. Thus the presentinvention provides a method of conducting pipe-laying operationsemploying a pipe-laying apparatus that includes a first pipe grippingassembly disposed on a pipe-laying path along which pipeline passesduring laying, and a second pipe gripping assembly disposed on thepipe-laying path, at least one of the first and second pipe grippingassemblies including both a pipe gripping arrangement for gripping apipe during pipe-laying and a cable gripping arrangement for gripping acable during an operation when a cable is connected to a length ofpipeline, the method including the steps of

-   -   gripping a pipe with the pipe gripping arrangement of the second        pipe gripping assembly,

gripping a pipe with the pipe gripping arrangement of the first pipegripping assembly,

releasing the grip of the second pipe gripping assembly on the pipe,

effecting relative movement, in a first direction along the pipe-layingpath, between the first and second pipe gripping assemblies,

gripping the pipe with the pipe gripping arrangement of the second pipegripping assembly,

releasing the grip of the first pipe gripping assembly on the pipe, and

effecting relative movement, in a second direction opposite to the firstdirection along the pipe-laying path, between the first and second pipegripping assemblies.

The method may also include the steps of

moving the second pipe gripping assembly relative to the first pipegripping assembly along the pipe laying path from a first position to asecond position whilst the grip of the second pipe gripping assembly isreleased,

and moving the second pipe gripping assembly relative to the first pipegripping assembly along the pipe laying path from the second position tothe first position whilst the grip of the second pipe gripping assemblyis gripping.

The operation may also involve feeding of a cable through the apparatus.There may for that purpose be provided a further cable grippingarrangement. Preferably both the first and second pipe grippingassemblies each include a cable gripping arrangement. The presentinvention provides a method of conducting pipe-laying operationsemploying a pipe-laying apparatus that includes a first pipe grippingassembly disposed on a pipe-laying path along which pipeline passesduring laying, and a second pipe gripping assembly disposed on thepipe-laying path, the first and second pipe gripping assemblies eachincluding both a pipe gripping arrangement for gripping a pipe duringpipe-laying and a cable gripping arrangement for gripping a cable duringan operation when a cable is connected to a length of pipeline, themethod including the steps of

gripping a cable connected to a length of pipeline with the cablegripping arrangement of the first pipe gripping assembly,

releasing the grip on the cable of the cable gripping arrangement of thesecond pipe gripping assembly,

effecting relative movement, in a first direction along the pipe-layingpath, between the first and second pipe gripping assemblies,

gripping the cable with the cable gripping arrangement of the secondpipe gripping assembly,

releasing the grip of the first pipe gripping assembly on the cable, and

effecting relative movement, in a second direction opposite to the firstdirection, along the pipe-laying path, between the first and second pipegripping assemblies.

As will be further understood, the apparatus of the invention ispreferably able to feed the combination of a cable and a pipe throughthe apparatus. During lowering of a pipeline, for example duringabandonment, the apparatus first lowers the pipe at the end of thepipeline and then lowers the cable connected to the end of the pipeline.Similarly during raising of a pipeline, for example during recovery, theapparatus first raises the cable connected to the end of the pipelineand then raises the pipe at the end of the pipeline.

Accordingly, the present invention provides a method of lowering apipeline employing a pipe-laying apparatus that includes a first pipegripping assembly disposed on a pipe-laying path along which pipelinepasses during laying, and a second pipe gripping assembly disposed onthe pipe-laying path, the first and second pipe gripping assemblies eachincluding both a pipe gripping arrangement for gripping a pipe duringpipe-laying and a cable gripping arrangement for gripping a cable duringan operation when a cable is connected to a length of pipeline, themethod including the steps of

-   -   gripping a pipe to which a cable is connected with the pipe        gripping arrangement of the first pipe gripping assembly,

effecting relative movement, in a first direction along the pipe-layingpath, between the first and second pipe gripping assemblies,

gripping the pipe with the pipe gripping arrangement of the second pipegripping assembly,

releasing the grip of the first pipe gripping assembly on the pipe,

-   -   effecting relative movement, in a second direction opposite to        the first direction along the pipe-laying path, between the        first and second pipe gripping assemblies,

gripping the cable connected to the pipe with the cable grippingarrangement of the first pipe gripping assembly,

releasing the grip of the second pipe gripping assembly on the pipe,

effecting relative movement between the first and second pipe grippingassemblies in the first direction,

gripping the cable connected to the pipe with the cable grippingarrangement of the second pipe gripping assembly,

releasing the grip of the first pipe gripping assembly on the cable, and

effecting relative movement between the first pipe gripping assembliesin the second direction. The aforementioned method may for example beused when laying or abandoning a pipeline. In relation to this aspect ofthe invention, the first and second pipe gripping assemblies areconveniently located such that on lowering the pipeline, the pipeline orcable first passes the first assembly and then the second assembly andalso such that movement in the first direction, when the first pipegripping assembly is gripping, corresponds to a lowering movement.

Also, the present invention provides a method of raising a pipelineemploying a pipe-laying apparatus that includes a first pipe grippingassembly disposed on a pipe-laying path along which pipeline passesduring laying, and a second pipe gripping assembly disposed on thepipe-laying path, the first and second pipe gripping assemblies eachincluding both a pipe gripping arrangement for gripping a pipe duringpipe-laying and a cable gripping arrangement for gripping a cable duringan operation when a cable is connected to a length of pipeline, themethod including the steps of

gripping a cable to which a pipe is connected with the cable grippingarrangement of the second pipe gripping assembly,

effecting relative movement, in a first direction along the pipe-layingpath, between the first and second pipe gripping assemblies,

gripping the cable with the cable gripping arrangement of the first pipegripping assembly,

releasing the grip of the second pipe gripping assembly on the cable,

effecting relative movement, in a second direction opposite to the firstdirection along the pipe-laying path, between the first and second pipegripping assemblies,

gripping the pipe connected to the cable with the pipe grippingarrangement of the second pipe gripping assembly,

releasing the grip of the second pipe gripping assembly on the cable,

effecting relative movement between the first and second pipe grippingassemblies in the first direction,

gripping the pipe connected to the cable with the pipe grippingarrangement of the first pipe gripping assembly,

releasing the grip of the second pipe gripping assembly on the pipe, and

effecting relative movement between the first pipe gripping assembliesin the second direction. The aforementioned method may for example beused when recovering, or otherwise raising, a pipeline. In relation tothis aspect of the invention, the first and second pipe grippingassemblies are conveniently located such that on raising the pipeline,the pipeline or cable first passes the second assembly and then thefirst assembly and also such that movement in the second direction, whenthe first pipe gripping assembly is gripping, corresponds to a raisingmovement.

Either or both of the first and second pipe gripping assemblies may movein order to effect the relative movement.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

By way of example an embodiment of the invention will now be describedwith reference to the accompanying drawings, of which:

FIG. 1 is a side elevation view of a pipe-laying vessel which may befitted with a pipe-laying apparatus embodying the invention,

FIGS. 2 a to 2 d illustrate schematically the principle of operation ofa pipe-laying apparatus embodying the invention and having two pipegripping assemblies,

FIGS. 3 a to 3 f illustrate schematically the operation of the pipegripping assemblies, and

FIGS. 4 a to 4 c illustrate a construction of one particular foof pipegripping arrangement, and

FIGS. 5 a and 5 b illustrate a construction of one particular form ofcable gripping arrangement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The pipe-laying vessel shown in FIG. 1 simply illustrates one particularpipe-laying vessel to which the invention may be applied. The vessel,indicated generally by the reference numeral 1, is a semi-submersiblevessel including a lay tower 2 pivotally mounted on the vessel. It isdesigned for J-laying of a pipeline. While the tower 2 is shown verticalit may be inclined at an angle of up to about 30° to the vertical (withthe top of the tower outboard of the vessel) when laying pipe accordingto the angle at which the pipe is required to leave the vessel. Thevessel shown in FIG. 1 is described in more detail in WO 00/05525, thecontent of which is incorporated herein by reference.

As will be understood by those skilled in the art, the vessel 1 is usedto lay pipeline by raising the length of pipe from the deck of thevessel into a position aligned with the tower and welding that length ofpipe to the end of the pipeline which is held on the tower. The pipelineis of a considerable weight, especially if pipe laying is being carriedout in deep water, and it is therefore necessary to hold up the end ofthe pipeline in the tower, both while the pipeline is stationary as anew section of pipe is added, and during lowering of the pipeline downthe tower 2. In the particular case of the vessel shown in FIG. 1,crawler track tensioners (not shown) are provided for holding, andcontrollably lowering the pipeline. The tensioners are not shown in FIG.1 but their approximate location on the tower is illustrated byreference numeral 3.

Other systems for holding, and controllably lowering, the pipelineduring pipe-laying may of course be provided in place of the crawlertrack tensioners. For example, a travelling block and clamp arrangementmay be employed. Also, while the vessel 1 is arranged to lay rigidpipeline during laying and lengths of pipe are welded to the end of thepipeline during laying, it may of course be arranged to lay flexiblepipeline that is stored on a reel.

In accordance with the invention, the vessel 1 is fitted with a specialpipe-laying apparatus 4 disposed on the tower 2 towards the lower end ofthe tower. The pipe-laying apparatus is provided in addition to thecrawler track tensioners or other systems for holding and lowering thepipeline and is primarily for use in situations such as abandonment andrecovery of the pipeline, as will be described further below. Theprinciple of operation of the pipe-laying apparatus 4 will now bedescribed with reference to FIGS. 2 a to 2 d.

The pipe-laying apparatus 4 comprises an upper pipe gripping assembly 5and a lower pipe gripping assembly 6. As will be understood, the pipegripping assemblies 5, 6 are disposed on the tower 2 of the vessel onthe path along which pipeline passes during laying. In FIGS. 2 a to 2 d,the pipeline is designated by reference numeral 7.

The upper pipe gripping assembly 5 is at a fixed position but the lowerpipe gripping assembly 6 is mounted for movement along the path of thepipeline. The dotted line in FIGS. 2 a to 2 d indicates a stationaryreference plane. A drive unit, for example a hydraulic piston andcylinder arrangement (not shown), is provided for moving the lower pipegripping assembly 6 towards and away from the upper pipe grippingassembly 5.

As also illustrated schematically in FIGS. 2 a to 2 d, each of the pipegripping assemblies 5, 6 is movable (by means not shown) between an openposition, such as that shown in FIG. 2 a for the upper assembly 5 and aclosed position, indicated by the letter c (in this and other Figures),such as that shown in FIG. 2 a for the lower assembly 6.

FIGS. 2 a to 2 d show one complete cycle of operation of the pipe-layingapparatus. In FIG. 2 a the lower pipe gripping assembly 6 is closed andgripping the pipe while the upper pipe gripping assembly 5 is open. Thelower assembly 6 is then moved downwards under the control of the driveunit to the position shown in FIG. 2 b; during this operation thepipeline 7 is lowered in a controlled manner. The upper pipe grippingassembly 5 is then closed and the lower pipe gripping assembly 6 issubsequently opened as shown FIG. 2 d. The lower pipe gripping assembly6 is then moved upwards by the drive unit back towards its originalposition as shown in FIG. 2 a; during this movement the assembly 6remains open and the pipeline 7 remains stationary, held by the uppergripping assembly 5. When the lower pipe gripping assembly 6 hasreturned to its original position it is closed, as shown in FIG. 2 a andthe upper pipe gripping assembly 5 is subsequently opened returning thearrangement to the same position as shown in FIG. 2 a, ready for thenext cycle of operations. Thus, in each cycle of operations the pipeline7 is lowered by a distance corresponding to the length of the stroke ofthe movement of the lower gripping assembly 6.

Whilst the effect of the cycle of operations shown in FIGS. 2 a to 2 dis to lower the pipeline it will readily be seen that by altering thesequence of operations, the pipeline 7 may be raised rather thanlowered.

The cycle of operations just described with reference to FIGS. 2 a to 2d is already known per se in the context of pipe laying where a pair ofsafety clamps is used to raise or lower a pipeline during specialoperations.

FIGS. 3 a to 3 f illustrate the special construction and operation ofthe pipe-laying apparatus embodying the invention. Referring first toFIG. 3 a, the upper pipe gripping assembly 5 is shown as including botha pipe gripping arrangement 51 and a cable gripping arrangement 52.Similarly the lower pipe gripping assembly 6 is shown as including botha pipe gripping arrangement 61 and a cable gripping arrangement 62. Thisenables the apparatus not only to be used to raise or lower a pipeline7, but also to raise or lower a cable 8 which is shown connected to theend of the pipeline 7 in FIGS. 3 a to 3 c. Such a cable connection wouldbe employed for example during abandonment or recovery operations.

In FIG. 3 a the pipeline 7 is shown passing through the apparatus and itcan be seen how by operating the upper and lower pipe grippingarrangements 51, 61, the pipe can be lowered in the manner alreadydescribed with reference to FIGS. 2 a to 2 d. FIG. 3 a shows thesituation where the end of the pipeline is connected to a cable and thepipeline 7 has almost passed through the upper gripping assembly 5. Atthis stage the cable gripping arrangements 52, 62 are inoperative. Asthe pipeline 7 is lowered further, however, to the position shown inFIG. 3 b, the upper cable gripping arrangement 52 is brought intooperation in place of the pipe gripping assembly 51 since it is nownecessary for the upper pipe gripping assembly 5 to grip cable, notpipe. Thus, as will be understood from the description above withreference to FIGS. 2 a to 2 d, the lower pipe gripping arrangement 61 isclosed, the upper cable gripping arrangement 52 is open and the pipeline7 is lowered. The upper cable gripping arrangement 52 is then closed,and the lower pipe gripping arrangement 61 is opened, as shown in FIG. 3c. The lower pipe gripping assembly 6 is then moved upwards by the driveunit back towards its original position. During this movement the lowerpipe gripping arrangement 61 remains open and the pipeline 7 and cable 8remain stationary, held by the cable gripping arrangement 52 of theupper gripping assembly. The lower pipe gripping arrangement 61 is thenclosed and the upper cable gripping arrangement 52 is opened, as shownin FIG. 3 d.

The pipeline 7 is lowered further by the lower pipe gripping assembly 6,to the position shown in FIG. 3 e. As before, the upper cable grippingarrangement 52 is then closed, the lower pipe gripping arrangement 61 isopened and the lower pipe gripping assembly 6 is moved upwards by thedrive unit back towards its original position. During this movement, thelower pipe gripping assembly 6 remains open and the pipeline 7 and cable8 remain stationary, held by the cable gripping arrangement 52 of theupper gripping assembly 5. FIG. 3 f shows the situation where it isnecessary to use the cable gripping arrangement 62 of the lower pipegripping assembly 6. The lower cable griping arrangement 62 is thenclosed. Following closure of the lower cable gripping arrangement 62,the upper cable gripping arrangement 52 is opened. The cable can then befurther lowered (or raised) by operating the cable gripping arrangement52, 62 of the gripping assemblies and reciprocating the upper grippingassembly as previously described with reference to FIGS. 2 a to 2 d.

Thus it may be seen how in accordance with the invention the pipegripping assemblies 5, 6 can be employed to raise or lower either thepipeline 7 or a cable 8 of much smaller diameter connected to thepipeline 7 and furthermore they are able to operate effectively even atthe junction of the pipeline 7 and the cable 8.

FIGS. 4 a and 4 b illustrate one particular construction of pipegripping arrangement that may be employed in the invention.

The pipe gripping arrangement 51 comprises generally wedge shaped pipecontacting elements 511 which can be moved by hydraulic pistonassemblies (not shown) between an open position, as shown in FIG. 4 aand a closed, pipe gripping position, (as shown in FIG. 4 b), alongguide members 512.

Each pipe contacting element 511 comprises an activating element 513 andgripping members 514. The gripping members 514 include teeth forengaging the outer surface of the pipe.

The activating element 513 has a friction pad 515 at one end forcontacting the outer surface of the pipe and roller bearings 516 at itsother end for contacting an opposing surface of the guide member 512.The activating element 513 can by hydraulic means (not shown) beextended into contact at one end with the outer surface of the pipe andat the other end with the guide member, thereby causing frictionalengagement between the pipe 7 and the friction pad 515.

While the pipe gripping arrangement 51 is not in use, it remains spacedfrom the pipe as shown in FIG. 4 a. The dimensions of the pipe grippingarrangement are chosen to match the diameter of the pipe being laid. Thepipe being laid may range from between 4 and 32 inches in diameter.

When the pipe gripping arrangement 51 is required to grip the pipe, thepipe contacting elements 511 are moved from the open position by thehydraulic piston assemblies 517 (not shown) in FIG. 4 a or FIG. 4 bdownwardly and radially inwardly towards the pipe along the guidemembers 512.

Thus the activating elements 513 and gripping members 514 may be drivento a position adjacent to the outer surface of the pipe, or they may bedriven out of a position adjacent to the outer surface of the pipetowards the open position.

When in a position adjacent to the outer surface, the activating element513 is extended until the friction pad 515 is forced into contact withthe pipe. Thus the activating elements 513 grip the pipe. The load ofthe pipeline on the pipe will be a downward load and therefore tends toencourage further downward movement of the pipe contacting elements 511.As a result of the generally wedge shape of the contacting elements 511and guide members 512, the activating element 513 and gripping members514 are forced towards the pipe and the teeth of the gripping membersengage with the outer surface of the pipe. The teeth make indentationsin the outer surface of the pipe and, by the penetration of the teethinto the pipe, affords extra grip to the contacting elements.

FIGS. 5 a and 5 b illustrate a construction of cable grippingarrangement 52.

The cable gripping arrangement 52 comprises cable contacting elements521 which, as shown in FIGS. 5 a and 5 b are wedge shaped and mountedfor inclined sliding movement along guide members 522, that slidingmovement being driven by drive units (hydraulic piston/cylinderassemblies) 523. Thus the cable contacting elements 521 may be driveninto contact with the cable to grip the cable by driving them downwardlyalong the guide members 522 (as shown in FIG. 5 b) or they may be drivenout of contact by driving them upwardly along the guide members 522 (asshown in FIG. 5 a). The load of the cable on the elements 521 will be adownward load and therefore tends to encourage further downward movementof the elements 521 resulting in stronger gripping of the cable.

It will be appreciated that various modifications may be made to theabove described embodiment without departing from the spirit of theinvention.

The number of pipe contacting elements 511 and cable contacting elements521 can be chosen to suit any particular application. Usually it will bepreferred to have between two and eight elements equiangularly spacedaround the pipe or cable.

The pipe gripping assemblies are able to be used with pipes having adiameter between 100 mm and 900 mm and cables with a diameter between 50and 200 mm. The pipe gripping assemblies may be used in pipe layingoperations wherein the load of the pipeline of the pipe grippingassemblies is between 500 and 2000 tons.

The maximum stroke of movement of the lower pipe gripping assembly 5relative to the upper pipe gripping assembly 6 is a matter of choice. Itwill usually be relatively small and may be of the order of 12 m; in oneparticular example it is about 8 m. In one particular arrangement thecable gripping arrangement of the lower pipe gripping assembly fitspartially inside the pipe gripping arrangement of the upper pipegripping assembly such that the stroke length of the cable grippingarrangement is slightly longer than the stroke length of the pipegripping arrangement.

By providing a pipe-laying apparatus of the kind described above, it ispossible to avoid using a powerful winch for abandonment and recoveryoperations. That can save cost because the winch is no longer requiredand can also improve safety because of the absence on the vessel of longwinch cables under high tension.

The pipe could be provided with an extra thick (5 mm) plastic pipecoating to allow limited indentation of the teeth of the pipe grippingmembers into the pipe coating layer to provide grip without prejudicingthe effectiveness of the corrosion-reducing function of thepipe-coating.

1. A method of conducting pipe-laying operations employing a pipe-layingapparatus that includes a first pipe gripping assembly disposed on apipe-laying path along which pipeline passes during laying, and a secondpipe gripping assembly disposed on the pipe-laying path, at least one ofthe first and second pipe gripping assemblies including both a pipegripping arrangement arranged to grip a pipe with a diameter duringpipe-laying and a cable gripping arrangement arranged to grip a cablewith a diameter, wherein the cable has a smaller diameter than the pipe,during an operation when the cable is connected to a length of pipeline,the method including the steps of gripping the pipe with the pipegripping arrangement of the second pipe gripping assembly, gripping thepipe with the pipe gripping arrangement of the first pipe grippingassembly, releasing the grip of the second pipe gripping assembly on thepipe, effecting relative movement between the first and second pipegripping assemblies, the relative movement in a first direction alongthe pipe-laying path, gripping the pipe with the pipe grippingarrangement of the second pipe gripping assembly, releasing the grip ofthe first pipe gripping assembly on the pipe, and effecting relativemovement between the first and second pipe gripping assemblies, in asecond direction opposite to the first direction, along the pipe-layingpath.
 2. A method according to claim 1, in which the pipe grippingarrangement includes pipe contacting elements including teeth that areable to make permanent indentations in the pipe when they grip the pipe.3. A method according to claim 1 in which over an outer surface of thepipeline there is provided a pipe coating layer to reduce corrosion anda further pipe coating layer and in which the pipe gripping arrangementincludes pipe contacting elements including teeth which are able toengage with the further pipe coating layer without prejudicing corrosionresistance of the pipe corrosion reducing layer.
 4. A method accordingto claim 3, in which the teeth of the pipe contacting elements are ableto indent progressively into the pipe coating layers, and if necessarythe pipe itself, in dependence on a gripping force required to grip thepipeline.
 5. A method according to claim 1, in which a verticalcomponent of the movement in the first direction is greater than ahorizontal component.
 6. A method according to claim 1, in which onlyone of the first and second pipe gripping assemblies move in order toeffect the relative movement between the first and second pipe grippingassemblies, the other pipe gripping assembly remaining in a fixedlocation.
 7. A method according to claim 1, in which both of the firstand second pipe gripping assemblies move in order to effect the relativemovement between the first and second pipe gripping assemblies.
 8. Amethod according to claim 1, employing an apparatus for laying apipeline from a vessel, the apparatus including a first pipe grippingassembly disposed on a pipe-laying path along which the pipeline passesduring laying, a second pipe gripping assembly disposed on thepipe-laying path, a drive unit for causing relative movement in adirection along the pipe-laying path between the first pipe grippingassembly and the second pipe gripping assembly, the first pipe grippingassembly including both a pipe gripping arrangement for gripping a pipeduring pipe-laying and a cable gripping arrangement for gripping a cableduring an operation when a cable is connected to a length of pipeline.9. A method of conducting pipe-laying operations employing a pipe-layingapparatus that includes a first pipe gripping assembly disposed on apipe-laying path along which pipeline passes during laying, and a secondpipe gripping assembly disposed on the pipe-laying path, the first andsecond pipe gripping assemblies each including both a pipe grippingarrangement arranged to grip a pipe with a diameter during pipe-layingand a cable gripping arrangement arranged to grip a cable with adiameter, wherein the cable has a smaller diameter than the pipe, duringan operation when the cable is connected to a length of pipeline, themethod including the steps of gripping the cable connected to the lengthof pipeline with the cable gripping arrangement of the first pipegripping assembly, releasing the grip on the cable of the cable grippingarrangement of the second pipe gripping assembly, effecting relativemovement, between the first and second pipe gripping assemblies in afirst direction along the pipe-laying path, gripping the cable with thecable gripping arrangement of the second pipe gripping assembly,releasing the grip of the first pipe gripping assembly on the cable, andeffecting relative movement between the first and second pipe grippingassemblies, in a second direction opposite to the first direction, alongthe pipe-laying path.
 10. A method of lowering a pipeline employing apipe-laying apparatus that includes a first pipe gripping assemblydisposed on a pipe-laying path along which the pipeline passes duringlaying, and a second pipe gripping assembly disposed on the pipe-layingpath, the first and second pipe gripping assemblies each including botha pipe gripping arrangement arranged to grip a pipe during pipe-layingand a cable gripping arrangement arranged to grip a cable during anoperation when a cable is connected to a length of pipeline, the methodincluding the steps of gripping the pipe to which the cable is connectedwith the pipe gripping arrangement of the first pipe gripping assembly,wherein the pipe has a first diameter and the cable has a seconddiameter, the second diameter being smaller than the first diameter,effecting relative movement between the first and second pipe grippingassemblies, in a first direction along the pipe-laying path, grippingthe pipe with the pipe gripping arrangement of the second pipe grippingassembly, releasing the grip of the first pipe gripping assembly on thepipe, effecting relative movement between the first and second pipegripping assemblies, in a second direction opposite to the firstdirection along the pipe-laying path, gripping the cable connected tothe pipe with the cable gripping arrangement of the first pipe grippingassembly, releasing the grip of the second pipe gripping assembly on thepipe, effecting relative movement between the first and second pipegripping assemblies in the first direction, gripping the cable connectedto the pipe with the cable gripping arrangement of the second pipegripping assembly, releasing the grip of the first pipe grippingassembly on the cable, and effecting relative movement between the firstpipe gripping assemblies in the second direction.
 11. A method ofraising a pipeline employing a pipe-laying apparatus that includes afirst pipe gripping assembly disposed on a pipe-laying path along whichthe pipeline passes during laying, and a second pipe gripping assemblydisposed on the pipe-laying path, the first and second pipe grippingassemblies each including both a pipe gripping arrangement arranged togrip a pipe during pipe-laying and a cable gripping arrangement arrangedto grip a cable during an operation when the cable is connected to alength of pipeline, the method including the steps of gripping the cableto which the pipe is connected with the cable gripping arrangement ofthe second pipe gripping assembly, wherein the pipe has a first diameterand the cable has a second diameter, the second diameter being smallerthan the first diameter, effecting relative movement between the firstand second pipe gripping assemblies, in a first direction along thepipe-laying path, gripping the cable with the cable gripping arrangementof the first pipe gripping assembly, releasing the grip of the secondpipe gripping assembly on the cable, effecting relative movement betweenthe first and second pipe gripping assemblies, in a second directionopposite to the first direction along the pipe-laying path, gripping thepipe connected to the cable with the pipe gripping arrangement of thesecond pipe gripping assembly, releasing the grip of the second pipegripping assembly on the cable, effecting relative movement between thefirst and second pipe gripping assemblies in the first direction,gripping the pipe connected to the cable with the pipe grippingarrangement of the first pipe gripping assembly, releasing the grip ofthe second pipe gripping assembly on the pipe, and effecting relativemovement between the first pipe gripping assemblies in the seconddirection.